Method of simultaneous stabilization and drying of wood



United States Patent Oil" 3,090,411 Patented May 21, 1963 3,090,411METHGD F SIMULTANEGUS STAEILIZATIGN AND DRYING 8F WGQD Flemming G..Iuncirer, Havndal, .lprgen Damgaard and Vagn Kingo Jacobson, Koge,Peter Honk, Heliernp and Eorgen Schmidt, Koge, Denmark, assiguors toJunckers Savvaerk A/S, Vaerftsvcj, Koge, Denmark No Drawing. Filed July26, 1961, Ser. No. 126,894 Claims priority, application Denmark July 29,1960 2 Claims. (Cl. led-320) It is known to reduce the tendency of woodto warp when the humidity of the air varies. This tendency tocontraction and bulging is one of the greatest drawbacks involved by theuse of wood. Since contraction and bulging are in different directionstangentially and radially to the annual rings of the wood and sincefurther the moisture content in the wood may vary from spot to spot,variations in the moisture content of the wood will involve warpings andtwistings of pieces already shaped.

In the course of time many proposals have therefore been made fortreating Wood in order to reduce its tendency to work as a consequenceof varying air humidity. A common feature of most stabilising methodsproposed in literature is at some stage of the treatment to subject thewood to a rather long period of impregnation with various chemicals suchas phenol resin. In fact, very substantial results of stabilisation havebeen obtained in this way, but the said methods have not been adopted toany wide extent in the wood working industry because they are rathercostly and therefore mean an unreasonable increase in the price of thewood.

It has also been tried to reduce the moisture content of fresh wood tol0l2% by pressing the wood mechanically between two plates heated to160' 250, using a pressure of 12-65 -kg./cm. :for %4 minutes. Thisdrying is, however, not sufficient to prevent the wood from working, butdrying beyond that range has been abandoned for fear that the woodshould lose some of its content of resin which is presumed to melt atthe said temperature. Furthermore, it is unavoidable that owing tohydrolysis there will be blotches in the outer layers of the fresh woodwhich face the pressing plates.

In order to avoid such blotches it has been found necessary in anothercase to air-dry wood veneer so much that it is subject to a linearcontraction of 13% prior to the press drying, at which it has then beenpossible to reduce the water content to 1-5 When subsequently the woodis carefully moistened to contain up to 6-8%, it will be stabilised, butit has in fact not been possible to avoid contraction to obtainstabilization without marring blotches.

The present invention has for its object to make it possible in pressdrying without formation of blotches to avoid contraction of the wood indirections at right angles to the direction of pressing, even in thecase of wood an inch thick or more.

According to the invention the wood, which is preferably available in athickness of at least mm, and which has a water content of 30% or morein which it has preserved its full widths, is subjected to a pressure of5-100 kg. per cm? at right angles to the direction of the fibres at atemperature of 110200 C. for a period of /25 hours until the watercontent of the wood has been reduced to less than 5% by weight. The saidperiod of pressure ensures the said drying of the wood even at such lowtemperatures as those indicated, at which the risk of blotches is verysmall.

By this treatment the wood is, as mentioned, only stabilized in adirection at right angles to the direction of pressure, that is, not inthe said direction itself. This is, however, less essential in severaluses. In the case of Z parquetry iloors, it is, for example, only thecontraction and bulging of the wood in one of the directions of aparquet block, namely the direction of width of the block, whichinvolves drawbacks in practice in the mounted parquetry floor.

By treating wood in the manner dealt with according to the invention thestabilization of the Wood in directions at right angles to the directionof pressure will be maintained, if only the water content of the wooddoes not later exceed about 25%. This means in practice that the woodpreserves its stability even though it is exposed to varying airhumidity which may go as far as up to about of relative humidity. If,however, the wood gets into contact with water, it will later be capableof contracting and bulging like untreated wood.

When beechwood from being in equilibrium at 20% relative humidity isbrought into equilibrium at 60% relative humidity, it will expand by5.7% in tangential direction. Under identical conditions the Woodtreated according to the invention and hot-pressed at right angles tothe direction of its fibres will be capable of expanding by about 2.2%.As a result, an anti-contraction eilect has been obtained equal to:

In conventional hot drying of crude beech blocks, reducing their watercontent to less than 5%, 10-15% of the blocks have had to be rejectedprior to press drying owing to warping, and the finished blocks havebeen produced in the running production with a width of 113 Bystabilizing and drying wood of the identical crude block width by themethod according to the invention it will be possible, owing to theeliminated contraction in width, to produce it in block widths of mm,which means an increase of 15% in area. With the use of the presentinvention considerable advantages are thus obtained in addition to asubstantially improved economy, since there will be practically nowarping nor twisting of the wood, nor will there be any perceptibledecrease in width.

Other advantages of the method according to the invention will beapparent from the following examples.

A piece of beechwood, 40 cm. long, 7.5 cm. wide and 3 cm. thick, waspressed in its green condition for two hours, the pressure applied tothe surface of the wood being 14 kg/cm. at a temperature of C. in thepressing plates.

When the wood was removed from the press, its water content haddecreased to 1%. Its thickness had decreased by 0.9 c. and its width by0.1 cm. Its length was unaltered.

A piece of ashwood :of identical dimensions was airdried to a watercontent of 30%. The wood was hotpressed under identical conditions andfor the identical time, and its water content was found to have beenreduced to 2 /2%. Its thickness had decreased by 0.7 cm. and its Widthby 0.1 cm.; its length was unaltered.

In a third case a piece of beechwood of identical dimensions asmentioned in the foregoing, but with red heart, was air-dried to a watercontent of 30% The wood was subsequently subjected to the samehot-pressing as was used in the two preceding examples, and when removedfrom the press the wood was 'found to have a water content reduced to3%. Its thickness had decreased by 0.7 cm. and its width by 0.1 cm. Itslength was unaltered.

If by further proceeding according to the invention the wood is fed tothe hot press in a boiled or thoroughly steamed condition, a reducedpressing time is sufiicient, just as the product obtained is without anyessential internal stresses.

Boiling or steaming of the wood can be performed in easiest manner byintroducing it into a steam oven or kiln which need not have anypressure substantially above atmospheric. The time the wood has to stayin the said oven depends on the thickness and state of the wood. In thecase of fresh beechwood of a thickness of 3 cm., 14% hours will besuitable.

According to the invention the wood has further prior to thehot-pressing being subjected to pressing or rolling at temperatures of100 C. and less and at pressures of 30-130 kg./cm. until the major partof the water contained in the pores of the wood has been squeezed out.By this preliminary treatment of the wood, which, Where fresh beechwoodis involved, may appropriately be for a :few minutes, that is, 2-10minutes, the advantage obtained is in addition to the aforesaidreduction of the pressing time a prompt removal of a substantial portionof the water-insoluble substances which would otherwise be apt todiscolor the wood when it is press-dried.

Furthermore, according to the invention the wood may be fed to the hotpress after prior absorption of ammonia in quantites of 1-3 percent byweight. The treatment of the wood with ammonia may take place atordinary ternperature for up to 6 hours in a kiln filled with vapors ofammonia. This absorption of ammonia will soften the wood so that it canbe compressed more at the hot pressing, and by means of the ammoniatreatment it is further possible to regulate the hue of the wood to thedesired degree. Incidentally, it is known to fume Wood by means ofammonia.

Prior to, during and after the absorption of ammonia the wood may betreated with aldehyde in a kiln in which the temperature is kept ataround 110-150 C. By sup plementing the ammonia treatment with thisaldehyde treatment the result obtained is that the stabilization aimedat by the hot pressing is further improved. Incidentally, the aldehydewill in known manner have a preserving efiect against fungi and insects.

Since several of thebest known and cheapest aldehydes, such asformaldehyde, only produce a stabilising efiect in acid condition, itmay be advantageous when using such aldehydes in combination with themethod accordingto the invention to add beforehand to the liquid orgaseous phase of the aldehyde small amounts of a suitable and such ashydrochloric acid, sulphuric acid, nitric acid or an acid salt such asammonium chloride, zinc chloride or aluminium chloride.

Compared with the hot-air drying in drying chambers as usually adoptedthe hot pressing itself means a very substantial economy in spacerequirements, frequently up to 90%, owing to the fact that the hotpressing can be carried through much faster than air drying.

What we claim and desire to secure by Letters Patent is:

1. In treating wood to stabilize its tendency to warp consequent tovarying air humidity, the improvement which comprises relieving the woodof its ability to contract tangentially tto its annual rings by applyingpressure of 5 to 100 kg. per om. at a temperature of 110-200 C. acrossthe width of the wood and only in a direction at right angles to itsfibers for a period of /2 to 5 hours until the water content of the woodhas been lowered to below 5% by Weight of the Wood, whereby the woodwill be stabilized against contraction in directions at right angles tothe direction of said hot pressing and will not warp with varying airhumidity.

2 In treating Wood to inhibit its tendency to warp consequent to varyingair humidity, the improvement which comprises relieving the wood ofability to contract tangentially to its annual rings by first adjustingthe water content of the wood to comprise about by weight and thereafterapplying pressure of 5 to 100 kg. per cm. at a temperature of 1l0200 C.across the width of the wood and only in a direction at approximatelyright angles to its fibers for a period of /2 to 5 hours until the watercontent of the wood has been lowered to below 5% by weight of the wood,whereby the wood will be stabilized against contraction in directions atright angles to the direction of said hot pressing and will not warpwith varying air humidity.

References Cited in the file of this patent UNITED STATES PATENTS1,356,015 Roy Oct. 19, 1920 1,697,248 Lauter Jan. 1, 1929 1,952,664Esselen Mar. 27, 1934 2,064,965 Will Dec. 22, 1936 2,136,730 SweetlandNov. 15, 1938 2,313,953 Loughborough .L Mar. 16, 1943 2,666,463 HeritageJan. 19, 1954

1. IN TREATING WOOD TO STABILIZE ITS TENDENCY TO WARP CONSEQUENT TOVARYING AIR HUMIDITY, THE IMPROVEMENT WHICH COMPRISES RELIEVING THE WOODOF ITS ABILITY TO CONTRACT TANGENTIALLY TO ITS ANNUAL RINGS BY APPLYINGPRESSURE OF 5 TO 100 KG. PER CM2. AT A TEMPERATURE OF 110-200* C. ACROSSTHE WIDTH OF THE WOOD AND ONLY IN A DIRECTION AT RIGHT ANGLES TO ITSFIBERS FOR A PERIOD OF 1/2 TO 5 HOURS UNTIL THE WATER CONTENT OF THEWOOD HAS BEEN LOWERED TO BELOW 5% BY WEIGHT OF THE WOOD, WHEREBY THEWOOD WILL BE STABILIZED AGAINST CONTRACTION IN DIRECTIONS AT RIGHTANGLES TO THE DIRECTION OF SAID HOT PRESSING AND WILL NOT WARP WITHVARYING AIR HUMIDITY.